1. Field of the Invention
The present invention relates to a circuit for correcting a differential offset of a transmission apparatus sending out a transmission signal of a telecommunication system.
2. Description of the Related Art
When using a quadrature modulation such as a binary phase shift keying (BPSK)/quadrature phase shift keying (QPSK) and such in a digital wireless technique, a differential offset amount of a transmission output circuit inputting a transmission signal as a differential signal to a quadrature modulator has a great deal of influence to a transmission performance. Therefore, the conventional technique feeds back a voltage of a differential output signal of a transmission output circuit, applies an analog/digital (AD) conversion and obtains a value of the output voltage, thereby correcting the differential offset based on the obtained value.
FIG. 1 is a diagram exemplifying a configuration of a conventional differential offset correction circuit.
A transmission digital signal generated at a transmission digital signal generation unit 10 is input to a primary signal differential output digital/analog converter (DAC) 12 of a transmission analog signal output unit 11-1. The transmission analog signal output unit 11-1 corresponds to an I channel-use when performing a quadrature modulation. When performing a quadrature modulation, it is also equipped in a Q channel-use transmission analog signal output unit 11-2. The configuration of the transmission analog signal output unit 11-2 is the same as that of the transmission analog signal output unit 11-1 and therefore the drawing thereof is omitted herein.
The primary signal differential output DAC 12 outputs a primary signal that is a digital signal as an analog difference voltage signal. The difference voltage signal output from the primary signal differential output DAC 12 is amplified by a fully differential amplifier 13. Respective signals of the differential voltage signal are input to a voltage value adjustment circuit comprising resistors R1 and R2 and a differential amplifier 14-1, and resistors R3 and R4 and a differential amplifier 14-2, followed by being set at a prescribed voltage and being output by way of an output control switch 15. The output voltages are indicated by the OIP on one side and by the OIM on the other. Because it is a differential voltage signal, the quadrature modulator is operated on the basis of the voltage difference between the OIP and OIM if an output signal is given to the quadrature modulator. Therefore, if there is a differential offset in the differential voltage signals OIP and PIM, the operation of the quadrature modulator is adversely influenced. This accordingly requires a control of the voltage values of the OIP and OIM by controlling the differential amplifiers 14-1 and 14-2.
The conventional technique shown in FIG. 1 is configured to input the outputs of the differential amplifiers 14-1 and 14-2 respectively to an offset detection-use AD converter (ADC) 17 by way of a comparator input select switch 16. The offset detection-use ADC 17 is an AD converter for converting the differential offset value of the analog differential voltage signal output from the differential amplifiers 14-1 and 14-2. The output of the offset detection-use ADC 17 is input to a control logic unit 18 which then inputs a reference voltage value, as a digital value, to the offset correction-use DAC 19-1 and 19-2, so as to minimize a differential offset amount as much as possible. The offset correction-use DACs 19-1 and 19-2 convert the digital voltage value obtained from the control logic unit 18 into analog voltage values and input to the respective terminals, on one side, of the differential amplifiers 14-1 and 14-2, respectively, as a reference voltage. Such an operation results in the reference voltage which is input to the differential amplifiers 14-1 and 14-2 being adjusted so as to minimize the differential offset, and therefore the voltage values of the differential voltage signal output from the differential amplifiers 14-1 and 14-2 are adjusted so as to minimize the differential offset.
As another conventional technique for reducing an adverse effect of a differential offset, there is one noted in a reference patent document 1, in which a method for adding correction data to transmission data in order to remove an adverse effect of a differential offset.
Patent document 1: Laid-Open Japanese Patent Application Publication No. H07-30596
In the case of the conventional technique shown in FIG. 1, there are various methods of an AD converter (i.e., the offset detection-use ADC 17), such as a consecutive conversion type, flash type, et cetera, in which a circuit area size, hence consumption current, generally increases with conversion speed. And, if a highly accurate offset adjustment is required, a minute adjustment is necessary after a correction because an error at an AD conversion and errors of a correction-use DAC (i.e., offset correction-use DAC 19-1 and 19-2) are accumulated.